U.S. patent number 6,274,112 [Application Number 09/456,367] was granted by the patent office on 2001-08-14 for continuous production of silica-based microgels.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Robert Harvey Moffett, Walter John Simmons.
United States Patent |
6,274,112 |
Moffett , et al. |
August 14, 2001 |
Continuous production of silica-based microgels
Abstract
A continuous process is provided for preparing silica microgels
using carbon dioxide as a gel initiator at a pressure of at least
about 172 kPa (about 25 psig). Consistent performance of microgel
can be produced with varying production rates.
Inventors: |
Moffett; Robert Harvey
(Landenberg, PA), Simmons; Walter John (Martinsburg,
WV) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
23812480 |
Appl.
No.: |
09/456,367 |
Filed: |
December 8, 1999 |
Current U.S.
Class: |
423/338 |
Current CPC
Class: |
C01B
33/143 (20130101) |
Current International
Class: |
C01B
33/143 (20060101); C01B 33/00 (20060101); C01B
033/12 () |
Field of
Search: |
;423/338,339
;516/110 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
584727 |
|
Oct 1959 |
|
CA |
|
WO 91/07350 |
|
May 1991 |
|
WO |
|
Primary Examiner: Griffin; Steven P.
Assistant Examiner: Johnson; Edward M.
Claims
What is claimed is:
1. A continuous process for preparing polysilicate microgels
comprising:
(a) contacting a feed stream comprising a silica source, wherein
the silica source is selected from the group consisting of an
aqueous solution of a water soluble silicate and a colloidal silica
sol, with a feed stream comprising carbon dioxide in a contacting
vessel to produce a mixture; and
(b) aging the mixture in an aging vessel to partially gel the
mixture to produce an aged mixture,
wherein the contacting step, the aging step, or both, is performed
at a pressure of at least about 172 kPa.
2. The process of claim 1 wherein the silica source is an aqueous
solution of a water soluble silicate and further comprising
diluting the aged mixture to a silica concentration of less than
10%, by weight, to produce a diluted mixture.
3. The process of claim 2 further comprising adjusting the pH of
the diluted mixture to a pH of less than 3 or greater than 9.
4. The process of claim 1 further comprising adjusting the pH of
the aged mixture to a pH of less than 3 or greater than 9.
5. The process of claim 1 wherein the silica source is an aqueous
solution of a water soluble silicate and the concentration of
silica in the feed stream comprising a silica source is in the
range of 0.5 to 15%, by weight.
6. The process of claim 5 wherein the concentration of silica in
the feed stream comprising a silica source is in the range of 1 to
10%, by weight.
7. The process of claim 1 wherein carbon dioxide is present in an
amount that is at least 80% of the stoichiometric amount needed to
neutralize the alkalinity of said silica.
8. The process of claim 1 wherein carbon dioxide is present in at
least a stoichiometric amount needed to neutralize the alkalinity
of said silica.
9. The process of claim 1 wherein the contacting step, the aging
step, or both are performed at a pressure in the range of about 344
kPa to 1380 kPa.
10. The process of claim 1 wherein the contacting vessel, the aging
vessel, or both are elastically deformable vessels.
11. The process of claim 10 further comprising deforming the
elastically deformable vessel from time-to-time to dislodge
deposits that form on the vessel walls.
12. The process of claim 1 further comprising dislodging deposits
that form on the walls of the contacting vessel, the aging vessel,
or both, by rapidly decreasing the pressure in the vessel from a
pressure of greater than about 172 kPa to ambient pressure.
13. A continuous process for preparing polysilicate microgels
comprising:
(a) contacting a feed stream comprising a silica source, wherein
the silica source is an aqueous solution of a water soluble
silicate, with a feed stream comprising carbon dioxide in a
contacting vessel to produce a mixture; and
(b) aging the mixture in an aging vessel to partially gel the
mixture to produce an aged mixture,
wherein the concentration of silica in the feed stream comprising
the silica source is in the range of 0.5 to 15%, by weight; carbon
dioxide is present in an amount that is at least 80% of the
stoichiometric amount needed to neutralize the alkalinity of said
silica, and said process is carried out at a pressure in the range
of about 344 kPa to 1380 kPa.
14. The process of claim 13 further comprising diluting the aged
mixture to a silica concentration of less than 5.0%, by weight, to
produce a diluted mixture.
15. The process of claim 14 further comprising adjusting the pH of
the diluted mixture to a pH of less than 3 or greater than 9.
16. The process of claim 13 further comprising adjusting the pH of
the mixture to a pH of less than 3 or greater than 9.
17. The process of claim 13 wherein carbon dioxide is present in at
least a stoichiometric amount needed to neutralize the silica
alkalinity.
18. The process of claim 13 wherein the feed stream comprising a
silica source and the feed stream comprising carbon dioxide are
simultaneously introduced into a mixing zone where the feed streams
converge at an angle not less than 30.degree..
19. The process of claim 18 wherein the contacting vessel, the
aging vessel, or both are elastically deformable vessels and
further comprising deforming the elastically deformable vessels
from time-to-time to dislodge deposits that form on the vessel
walls, and purging the deposits from the vessels.
20. A continuous process for preparing polysilicate microgels
comprising:
(a) contacting a feed stream comprising a silica source, wherein
the silica source is an aqueous solution of a water soluble
silicate, with a feed stream comprising carbon dioxide in a
contacting vessel to produce a mixture having a pH in the range of
6.5 to 7.5;
(b) aging the mixture in an aging vessel to partially gel the
mixture to produce an aged mixture;
(c) diluting the aged mixture to a silica concentration of less
than 5%,
wherein carbon dioxide is present in an amount that is at least the
stoichiometric amount needed to neutralize the alkalinity of said
silica, and said process is carried out at a pressure in the range
of about 344 kPa to 1380 kPa.
Description
FIELD OF THE INVENTION
This invention relates to a continuous process for preparing
silica-based microgels whereby consistent quality of the microgel
can be achieved with varying production rates.
DESCRIPTION OF THE RELATED ART
Polysilicate microgels (i.e., aqueous solutions formed by the
partial gelation of a soluble silica) are well known in the art.
These microgels can be prepared by partial gelation of an alkali
metal silicate by mixing the silicate with a gel initiator, aging
the mixture for a short time, and then stopping further gelation by
diluting the mixture. Gel initiators are also referred to as
"neutralizing agents" and/or "activating agents". Mineral acids and
alum are the most commonly employed gel initiators. Resulting
microgels have commercial utility as a drainage and retention aid
in paper making, as a flocculation agent in potable water
purification plants, and in similar applications.
Several practical factors currently limit commercial use of
polysilicate microgels, although they are excellent flocculents and
environmentally benign. For example, microgel solutions are
necessarily dilute, making it impractical to ship large volumes
long distances. Therefore, microgels are typically produced at a
field site by the user. Microgels also are prone to gel and to form
silica deposits in equipment used to prepare the product. These
problems can be overcome by equipment design and trained personnel
in a factory environment, but present greater difficulty in field
applications where the equipment should be relatively easy to
operate and maintain.
Batch and continuous processes have been developed through the
years to produce silica microgels. However, consistency in product
performance has been found to vary considerably from batch-to-batch
and even over relatively short periods of time with a continuous
process.
Performance of silica microgels as flocculents has been shown to be
highly dependent upon growing the silica microgels to the proper
size before use. Two of the most important factors that affect the
size of the microgel produced are pH during partial gelation of the
silicate and reaction time, which includes mixing time, but is
primarily aging time, or time until dilution of the silica microgel
solution.
During partial gelation of the silica, pH is difficult to control
when using a strong acid, such as sulfuric acid, as the gel
initiator because typically about 85% of the silicate alkalinity is
neutralized so the microgel can be used quickly after preparation.
Small changes in the amount of initiator result in large variations
in pH, which in turn result in changes in the microgel size and in
microgel performance.
In a batch reactor, reaction time can be easily changed. However in
a continuous reactor, reaction time is determined by the flow rate
of the reactants and the volume of the reactor. If the flow rates
of the reactants are changed, for example, to meet a change in
demand rate for the silica microgel, then reaction time, size of
the microgel formed, and therefore, microgel performance, will
vary.
Moffett and Rushmere, in U.S. Pat. Nos. 5,279,807; 5,503,820;
5,648,055; and 5,853,616 disclose improved continuous processes for
preparing polysilicate microgels wherein silica deposition is
greatly reduced by mixing a soluble silicate solution and a gel
initiator under specific conditions.
Moffett, Simmons, and Jones in U.S. patent application Ser. No.
09/119,468, filed Jul. 20, 1998 disclose a continuous process for
preparing polysilicate microgels wherein elastically deformable
vessels are incorporated into the process. Such vessels enable
dislodging of deposits formed on vessel walls.
While the designs taught in these patents result in much decreased
deposition, and have found commercial utility, there remains lack
of consistent quality when production rate of microgels is varied.
When flow rates of reactants are changed to meet varying demand
production rates, aging time changes and hence, size of the
microgel formed and performance can vary. Thus, the user must
accommodate for varying performance.
A continuous process that could reliably produce polysilicate
microgels of consistent quality having consistent performance at
different production rates in the same equipment to meet varying
customer demands would have high utility.
SUMMARY OF THE INVENTION
The present invention provides a continuous process for preparing
polysilicate microgels comprising:
(a) contacting a feed stream comprising a silica source, wherein
the silica source is selected from the group consisting of an
aqueous solution of a water soluble silicate, a colloidal silica
sol, and combinations thereof, with a feed stream comprising carbon
dioxide in a contacting vessel to produce a mixture; and
(b) aging the mixture in an aging vessel to partially gel the
mixture to produce an aged mixture,
wherein the contacting step, the aging step, or both, are performed
at a pressure of at least about 172 kPa (about 25 psig).
The stream comprising carbon dioxide may contain free carbon
dioxide, typically in the form of a gas or liquid, or in the form
of a material that will release carbon dioxide under reaction
conditions. Mixtures of carbon dioxide with other components are
also contemplated.
The present invention and its particular embodiments provide
advantages in a continuous process for preparing silica microgels,
which include better pH control during the step where the feed
streams are contacted; more consistent microgel size and
performance; elimination of strong mineral acids in the process,
which provides safety benefits as well as lower equipment costs;
improved ability to remove silica deposits; and efficient system
purge in the event water supply is lost.
DETAILED DESCRIPTION OF THE INVENTION
Polysilicate microgels are aqueous solutions formed by the partial
gelation of a silica source, for example, a water soluble silicate,
a colloidal silica sol, or combinations thereof.
Water soluble silicates include alkali metal silicates and
polysilicates, such as sodium silicate, having in its most common
form one part Na.sub.2 O to 3.3 parts SiO.sub.2 by weight.
Microgels formed from soluble silicates typically are composed of
water and linked silica particles having a diameter of 1 to 5 nm
and a surface area of at least 500 m.sup.2 /g, more typically of at
least 1000 m.sup.2 /g. The particles are linked together during
preparation (i.e., during partial gelation) to form aggregates
having three-dimensional networks and chains. Preferably, the
silica source is an aqueous solution of a water soluble
silicate.
Colloidal silica sols are commercially available, for example, from
E. I. duPont de Nemours and Company, Inc., sold under the name
Ludox.RTM. Colloidal Silica. Silica sols useful in this invention
are composed of water and discreet silica particles having a
diameter of 4 to 60 nm, preferably less than 50 nm. The sol
particles also link together during partial gelation to form
aggregates having three-dimensional networks and chains. Microgels
formed from silica sols will typically have a surface area in the
range of about 50 to 750 m.sup.2 /g.
At a pH below 5, polysilicate microgels sometimes are referred to
as polysilicic acid microgels. As the pH value is raised, these
products can contain mixtures of polysilicic acid and polysilicate
microgels, the ratio being pH-dependent. As used herein, the term
"polysilicate microgel" or "silica microgel" includes such mixtures
of polysilicic acid and polysilicate microgels.
Polysilicate microgels frequently are modified by incorporating
aluminate ions into their structure. The aluminum may be present
throughout the polysilicate aggregates, or only on their surface,
depending on where the aluminum source is added to the process.
Aluminum may be added to increase the rate of microgel formation,
and thus to decrease the aging time. Aluminum added as aluminate
also allows the microgel to retain its charge at low pH conditions.
Silica sols may have aluminum incorporated in the sol particles. As
used herein, the term "polysilicate microgel" or "silica microgel"
includes polysilicate microgels containing aluminum, which are
sometimes referred to in the art as polyaluminosilicate
microgels.
Contacting
In the present invention, a feed stream comprising a silica source,
the "silica feed stream", is contacted with a feed stream
comprising carbon dioxide. The silica source is selected from the
group consisting of an aqueous solution of a water soluble
silicate, a colloidal silica sol, and combinations thereof. The
term "contacting" refers to combining the two feed streams such
that the streams become mixed. Mixing is typically accomplished by
turbulent flow of the feed streams. The silica feed stream can
comprise any conventional water soluble silicate solution and/or
colloidal silica.
When the silica feed stream comprises a water soluble silicate
solution, the silica stream should have a silica concentration in
the range of 0.5 to 15%, preferably 1% to 10%, and most preferably
1 to 5%, by weight. The microgel generally will be formed too
slowly for practical use at concentrations below 0.5%. Above 15%
silica, the rate of gelation is too fast to effectively control
when using a water soluble silicate. Commercial silicate solutions
having higher silica concentrations can be used with appropriate
dilution by adding water to reduce the silica concentration.
When the silica feed stream comprises a colloidal silica sol, in
the absence of a water soluble silicate, the silica stream can be
used without dilution. The silica concentration in the feed stream
can be the same as the silica concentration in the silica sol, or
less. Preferably the silica concentration in the silica feed stream
is in the range of 15% to 50%, by weight, when a colloidal silica
sol is used, in the absence of a water soluble silicate.
Contacting the feed streams can be carried out in any suitable
contacting vessel, such as a tank, pipe, tube, hose, continuous
stirred tank, plug flow reactor, tubing, or combinations thereof.
The term "vessel" denotes a hollow subject used for fluid,
especially liquid.
The silica feed stream is contacted with a feed stream comprising
carbon dioxide as a gel initiator in a continuous process, which
initiates formation of the microgel. While it is preferred to feed
carbon dioxide as free carbon dioxide in the form of a gas or
liquid in the feed stream, the feed stream can also contain carbon
dioxide in the form of a material that will release carbon dioxide
under reaction conditions, for example, sodium bicarbonate. Other
components, liquids or gases may also be present in the carbon
dioxide feed stream.
The flow rates of the silica and carbon dioxide feed streams may be
controlled volumetrically (typically within the pH range of 6 to
10) due to the buffering effect of resulting carbonates. Volumetric
control offers the advantage of avoiding pH sensors, which may
require frequent cleaning, calibration, and replacement.
Additional gel initiators may be added, for example, aluminum
compounds, especially when preparing polyaluminosilicate microgel
solutions, which may lower the pH. Other gel initiators may also be
fed to the reactor, either with the carbon dioxide feed stream, or
as a separate stream. These include for example, inorganic and
organic acids, such as sulfuric and acetic acids, acid salts, such
as borax, sodium bisulfite, ammonium sulfate, alkali metal salts of
amphoteric metal acids, such as sodium aluminate and certain
organic compounds, such as organic anhydrides, amides and esters. A
more complete list of gel initiators is provided in Rushmere, U.S.
Pat. No. 4,954,220, incorporated herein by reference.
The contacting step, subsequent aging step or both steps should be
performed at a pressure of at least about 172 kPa (about 25 psig),
and typically less than 13,800 kPa (about 2000 psig), the upper
limit set more by practicality and economics than by process
limitations. Preferably the contacting step is performed at a
pressure in the range of about 344 to 1380 kPa (about 50 to 200
psig). Surprisingly it has been found that at the elevated
pressure, continuous production of silica microgels having a
consistent size is achieved, even at varying flow rates. For
example, production rates can be varied over a 3-fold range without
the need to vary either silica concentration or CO.sub.2 flow
rate.
The carbon dioxide feed stream is contacted with the silica feed
stream such that the amount of carbon dioxide added ranges from
slightly less than stoichiometric to an excess, based on the amount
needed to neutralize the alkalinity of the silica feed stream.
Slightly less than stoichiometric will typically mean at least 80%
of the stoichiometric amount, and preferably at least 90% of the
stoichiometric amount needed to neutralize the silica alkalinity.
The amount of carbon dioxide added can be less than 80% of the
stoichiometric amount needed to neutralize the silica alkalinity
when carbon dioxide is used in combination with other gel
initiators. Preferably there is at least a stoichiometric amount
added for more consistent pH control. More preferably CO.sub.2 is
added in an amount corresponding to 100 to 500% of the
stoichiometric amount needed to neutralize the silica
alkalinity.
By silica alkalinity, it can be the alkalinity of an aqueous
solution of a water soluble silicate, e. g., of a solution of an
alkali metal silicate, such as sodium silicate. These solutions are
basic and gel initiators are typically acidic. Water soluble
silicates are distinguished by their ratio of silica to alkali,
wherein the alkali is of the formula M.sub.2 O and M is typically
Na, K, or NH.sub.4. Alternatively, silica alkalinity can mean the
alkalinity of a colloidal silica sol. In a silica sol, the silica
particles are dispersed in an alkaline medium, which stabilizes the
particles to gelation. The alkaline medium can contain, for
example, sodium or ammonium hydroxide.
Preferably carbon dioxide will be used in the absence of other gel
initiators and the feed rate of carbon dioxide will be in excess of
the solubility of carbon dioxide in water at the given pressure and
temperature. As temperature increases, solubility of carbon dioxide
decreases. The temperature for carrying out either step is
typically in the range of from 0.degree. C. to 50.degree. C.
Optionally, an aluminum salt or an alkali metal aluminate, is
conveniently added as a soluble component in the silicate solution,
or may be added as a separate stream to the mixture. Excellent
polyaluminosilicate microgels contain an Al.sub.2 O.sub.3
/SiO.sub.2 mole ratio in the range of 1:1500 to 1:25, preferably
1:1250 to 1:50. Generally up to 25% of surface silicon can be
replaced by aluminum.
While any conditions may be employed in practicing the invention
for contacting the feed streams, turbulent conditions are
preferred, such that the feed streams are contacted at a Reynolds
number of at least 1,000, and preferably greater than 6,000.
After the contacting step, when the silica feed stream comprises a
water soluble silicate solution, the mixture should have a silica
concentration of 0.5 to 15 wt %, preferably 1 to 10 wt %, most
preferably 1 to 5 wt %. When the silica feed stream comprises a
colloidal silica sol in the absence of a water soluble silicate,
the mixture can have a higher silica concentration, i. e., a silica
concentration equal to the silica concentration in the silica sol
or less, preferably a silica concentration in the range of 15% to
50%, by weight. The pH should be in a range of 6 to 10, preferably
6.5 to 7.5, when carbon dioxide is used in the absence of
additional gel initiators.
Aging
The mixture is aged in an aging vessel for a time sufficient to
achieve the desired level of partial gelation, which usually takes
at least 10 seconds and generally does not take longer than 15
minutes. Partial gelation produces an aged mixture, which is an
aggregate of three-dimensional networks and chains of high surface
area silica particles known in the art as polysilicate microgels.
This aging step is preferably performed at a pressure greater than
about 172 kPa (about 25 psig) and typically less than 13,800 kPa
(2000 psig). Preferably the mixture is aged at a pressure in the
range of 344 to 1380 kPa (50 to 200 psig).
The extent of desired partial gelation varies with the selected
ingredients and the application, but generally is achieved within
10% to 90% of the time that produces complete gelation. Thus, an
artisan can readily determine gel time, and adjust the silica
concentration to achieve the desired partial gelation.
Alternatively, length and/or diameter of the aging vessel,
temperature, and flow rates, may be optimized for a particular
application. Temperature during aging typically is in the range of
0.degree. C. to 50.degree. C.
In a continuous process, aging occurs as the mixture passes through
an aging vessel, which is typically an elongated vessel, and is
essentially completed when the mixture reaches the vessel
discharge. The aging vessel can be any suitable vessel, such as a
pipe, tube, hose, plug flow reactor, tubing, or combinations
thereof. The aging vessel typically has a constant diameter, with
the diameter and length being selected to provide the needed
residence time for the mixture to "age" to the desired extent. A
typical aging vessel has a diameter in the range of 0.5 cm to 25 cm
(1/4 to 10 inches), and a length of 60 cm to 150 m (2 to 500 feet),
to provide a residence time of 10 seconds to 15 minutes. There
generally is no advantage to employing a residence time longer than
15 minutes.
The contacting and/or aging vessel is preferably an elastically
deformable vessel (e. g., a pipe or tube), as described in U.S.
patent application Ser. No. 09/119,468, filed Jul. 20, 1998.
Therein is described a continuous process for preparing
polysilicate microgels wherein the elastically deformable vessel is
described as being temporarily deformed from time-to-time to
dislodge deposits that form on the vessel walls. The vessel is
constructed of a material having (i) an elasticity greater than
that of silica deposits, and (ii) surface characteristics such that
deformation of the vessel will overcome adhesive forces between the
vessel and the deposits, thereby causing the deposits to be
dislodged when the vessel is deformed. Use of carbon dioxide at
pressures of at least about 172 kPa (about 25 psig) is especially
beneficial for eliminating silica deposits. The dislodged deposits
are purged from the vessel by the mixture of the silica stream and
carbon dioxide as the mixture continuously passes through the
vessel. As the deposits are composed of silica, there is no need to
segregate and remove them from the mixture exiting the vessel for
many applications. Advantages of using deformable vessels are
especially apparent when applied to the contacting and early aging
period of the process where deposits are particularly prone to
form.
While the use of pressure in the present invention will contribute
to deforming a deformable vessel, when such a vessel is used, other
means of deforming the contacting and/or aging vessel may also be
used. These include mechanical means and vibrational means.
Mechanical means include squeezing, stretching, or bending and
releasing the walls of a vessel by a roller, press or other
mechanical device, or by varying external pressure on a vessel by a
surrounding fluid. Vibrational means includes use of sonic or
ultrasonic sound waves.
Use of carbon dioxide also greatly aids in purging the reaction
system, which includes the contacting vessel and the aging vessel,
to dislodge deposits that form on the vessel walls. That is,
purging, or flushing, occurs when the pressure is reduced from the
reaction pressure of at least about 172 kPa (about 25 psig) to
ambient. The process of this invention optionally further comprises
a step that involves purging of the contacting and/or aging vessels
by dislodging deposits that form on the vessel walls by rapidly
decreasing the pressure in the vessels. The reduction in pressure
causes a change in solubility of carbon dioxide, which generates
large volumes of gas that rapidly purge the system. This is
particularly advantageous when using a deformable vessel as the
contacting vessel and/or the aging vessel.
Industrial Use
The aged mixture or polysilicate microgel is generally treated to
arrest, or minimize, further gel formation. When the silica feed
stream comprises a solution of a water soluble silicate, this
treatment may be a simple dilution step that reduces the silica
concentration to less than about 10%, preferably less than 5%, and
most preferably less than 2%, by weight, to produce a diluted
mixture, or diluted polysilicate microgel. When the silica feed
stream comprises a colloidal silica sol in the absence of a water
soluble silicate, a dilution step may also be used. Alternatively,
the treatment may be a pH adjustment step, or a combination of
dilution and pH adjustment steps, whereby gelation is halted or
retarded or both. In a pH adjustment step, pH is typically adjusted
to less than 3 or greater than 9 to minimize further gel formation.
Other techniques known in the art may be selected to arrest gel
formation as well.
The treated microgel, that is, after dilution or pH adjustment may
then be stored or consumed in its intended use. Alternatively, if
the microgel is consumed immediately, or if further gelation will
be within acceptable limits for the intended application, it may
not be necessary to dilute or adjust pH of the microgel. If
desired, the aged microgel may be filtered to remove unacceptably
large silica deposits that were dislodged while practicing the
invention.
Polysilicate microgels prepared in accordance with the invention
may be used in conventional applications consuming such microgels,
as well as in new applications rendered practical because the
microgels can be reliably produced in the field. For instance, the
microgels may be used as a flocculating agent to remove solids from
aqueous suspensions, or as a paper retention aid, frequently in
conjunction with other polymers and/or chemicals used for that
purpose.
Having described the invention, it now will be illustrated, but not
limited, by the following examples.
EXAMPLES
Example 1
This example demonstrates how the silica stream flow rate may be
varied resulting in aging times that vary without substantially
affecting the microgel size when using CO.sub.2 as a gel initiator
at a pressure of 75 psig. This example also demonstrates how the
gel initiator to silica ratio may be significantly changed without
substantially affecting the microgel size when using CO.sub.2 as a
gel initiator at a pressure of 75 psig.
A polysilicate microgel was prepared by mixing a solution of sodium
silicate with carbon dioxide. The sodium silicate solution had a
3.22 ratio of Na.sub.2 O:SiO.sub.2 and contained 28.5 wt %
SiO.sub.2. The silicate solution was diluted in-line with a
sufficient amount of water to lower the SiO.sub.2 concentration to
2.4 wt % SiO.sub.2, which was then fed continuously into 91.4 m
(300 feet) of an elastically deformable vinyl hose having an
internal diameter of 3.8 cm (1.5 inches). The flow rate of the
dilute silicate solution was varied to achieve different reaction
times. As used herein, "reaction time" means the theoretical aging
time, or "aging time" based on the flow rate of the dilute silicate
solution. Carbon dioxide (CO.sub.2) was fed to the hose at a flow
rate of 340 slpm. The amount of CO.sub.2 added ranged from slightly
less than stoichiometric to an excess, based on the amount needed
to completely neutralize the silicate alkalinity, depending on the
flow rate of the silicate solution. Pressure within the hose was
maintained at 516 kPa (75 psig) by a control valve located at the
discharge end of the hose. The pH and the time for the inception of
visible gelation, that is, "gel time" of the 2.4 wt % silica
microgel product were measured at the discharge end of the
hose.
Samples of the silica microgel product were prepared for viscosity
measurements by immediately diluting the 2.4 wt % microgel product
to 1 wt % SiO.sub.2 and lowering the pH to 2. Viscosity was
measured using a Cannon Fenske size 50 viscometer tube. Average
microgel size (diameter in nm) was estimated by comparing viscosity
measurements with viscosity measurements of a series of silica
microgels for which particle sizes were measured using dynamic
light scattering analysis. Viscosity measurements of the series of
microgels were correlated with particle size measured using the
light scattering analysis.
Dynamic light scattering analysis was performed using a Brookhaven
Instrument light scattering goniometer, model BI-200SM.
Measurements of the microgels were conducted at room temperature
using an argon-ion laser with a wavelength of 488 nm operating at
200 mW power. Light scattering intensity measurements were made at
different angles and the data were analyzed using a Zimm plot.
Results are provided in Table 1.
TABLE 1 Dilute silicate Aging Gel Est. Avg. solution, time, time,
Viscosity, Microgel l/m (gpm)* min. pH min. % DI H.sub.2 O
Diameter, nm 46.9 (12.4) 2.2 7.1 1.5 39.0 (10.3) 2.7 6.8 1.7 124 65
31 (8.2) 3.4 6.7 1.9 23 (6.2) 4.4 6.8 2.0 122 60 16 (4.1) 6.7 6.8
1.7 *1/m = liters per minute; gpm = gallons per minute
As can be seen in Table 1, gel times and pH of the microgel were
very consistent indicating consistent microgel size.
Comparative Example
This example demonstrates how varying the aging time when using
sulfuric acid as a gel initiator results in significant changes in
the microgel size and hence its properties. This example also
demonstrates how small changes in the gel initiator to silica ratio
when using sulfuric acid results in large changes in the microgel
size and properties.
For comparison, a polysilicate microgel was prepared by mixing 5N
H.sub.2 SO.sub.4 with 983 grams of dilute sodium silicate to
provide a microgel solution containing 2.4 wt % SiO.sub.2. Two
separate runs were performed, at 41.5 ml and 41.0 ml H.sub.2
SO.sub.4, respectively. The mixture was allowed to age to provide
similar reaction times as in Example 1. The pH was measured after 1
minute of aging. Samples were withdrawn from the mixture and
diluted to 1 wt % SiO.sub.2 and adjusted to pH 2 at the noted aging
times. Viscosity and estimated diameter for the aged samples were
determined as in Example 1. Gel time for each run is also
provided.
TABLE 2 5N Gel Aging Est. Avg. H.sub.2 SO.sub.4, pH @ time, time,
Viscosity Microgel ml 1 min. min. min. % DI H.sub.2 O Diameter, nm
41.5 8.77 7.5 2.2 120 50 3.4 132 90 6.7 201 >160 41.0 8.89 11.25
2.2 117 40 3.4 124 65 6.7 152 130
As can be seen from Table 2, use of sulfuric acid as the gel
initiator to prepare silica microgels provided microgel products
with significantly different viscosities and particle sizes at
different aging times. Also, a small difference in the amount of
sulfuric acid resulted in a large difference in gel time and
differences in viscosity and particle size. Comparing with the
results in Table 1, it can be seen that use of carbon dioxide as
the gel initiator provided a much more consistent product.
In Example 1 the amount of initiator (CO.sub.2) changes over a
three-fold range relative to the silicate and little variation in
viscosity and average microgel diameter was observed. In contrast,
in the Comparative Example, the amount of initiator (sulfuric acid)
changes only by 1.2% relative to the silicate and large changes in
viscosity and average microgel diameter were observed.
Example 2
This example demonstrates how a more consistent product is produced
by conducting the contacting and aging steps at pressures greater
than 172 kPa (about 25 psig).
The process of Example 1 was repeated at varying pressures. A
sodium silicate solution having a 3.22 ratio of Na.sub.2
O:SiO.sub.2 containing 28.5 wt % SiO.sub.2 was diluted in-line with
a sufficient amount of water to lower the SiO.sub.2 concentration
to 2.1 wt % SiO.sub.2. CO.sub.2 was added at a rate of 270 slpm.
The flow rate of dilute SiO.sub.2 was 28 liters per minute (7.5
gpm). The CO.sub.2 and silicate solution were continuously fed into
91.4 m (300 feet) of vinyl hose having an internal diameter of 3.8
cm (1.5 inches). Pressure within the hose was maintained by a
control valve located at the discharge end of the hose. The pH and
the gel time of the 2.1 wt % sol were measured at the discharge end
of the hose.
TABLE 3 Pressure, Gel time, kPa (psig) pH min. 619 (90) 6.9 1.2 516
(75) 6.9 1.3 344 (50) 6.9 1.3 172 (25) 7.2 2.2 103 (15) 7.8 2.8
As can be seen from Table 3 gel times and pH were most consistent
at pressures of about 172 kPa (about 25 psig) and above. Consistent
gel times and pH indicate consistent microgel size.
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